Endothelial cell genes are likely to be differentially expressed among patients with sickle cell disease. Variable expression of selected genes may, by modulating the response of the endothelium to sickle cells, leukocytes, growth factors, chemokines, cytokines, adhesion molecules and hemostasis, account for some heterogeneity of vasoocclusive disease. In addition, polymorphisms in genes expressed in endothelial cells and other tissues may alter gene expression or the gene product. Single nucleotide polymorphisms (SNPs) that segregate with selected features of the disease may mark important genes for further study. We plan a twofold approach to the question of genetic modulation of defined phenotypes of sickle cell disease. First, using microarrays containing cDNA of human microvascular endothelial cell expressed genes, we will profile the pattern of gene expression in endothelial cells obtained from patients with sickle cell anemia and HbSC disease who have defined phenotypes; we will also ascertain if there is enhanced responsiveness of their endothelial cells to biological stimuli such as TNF , IL1 or LPS. Second, using genomic DNA, we will search for SNPs in genes we hypothesize could play a role in phenotypic heterogeneity of sickle cell anemia and in genes whose endothelial cell expression differs in patients with and without designated phenotypes. In the first phase of our SNP studies, we will use banked DNA from more than 2000 patients with sickle cell disease who participated in the NHLBI-supported Cooperative Study of Sickle Cell Disease and study pooled DNA from patients with defined phenotypes to establish an association between a SNP and a phenotype. In a second phase, we will confirm positive findings in individual samples from these pools. A third phase will involve a search for SNPs in an independent population of family triads with a sickle cell disease proband having a defined phenotype to establish association and linkage of a SNP and phenotype. Our prime objectives are to learn if endothelial cell gene expression or polymorphisms in endothelial cell-expressed genes correlate with phenotypes of sickle cell disease and discover SNPs that segregate with defined phenotypes of sickle cell anemia. We hope to develop insights into the relationships of endothelial-expressed genes and clinical features of sickle cell disease and find SNPs that mark modifiers of disease severity.